卸载型异常高压地层压力预测方法

南海乐东区域是典型的卸载型高压区域，常规地层压力预测方法在该区域的适用性较差，现场钻井作业面临井涌、井喷等风险。为建立适用于乐东区域的地层压力预测模型，利用人工岩心开展室内声波测试实验，分别建立了声波波速与有效应力、岩性和泥质含量的函数关系。实验结果表明，声波波速与有效应力呈指数型关系，声波波速与孔隙度和泥质含量的平方根呈现线性关系。利用线性叠加原理，建立了改进的Bowers地层压力预测模型。改进的Bowers模型克服了原模型中只针对泥页岩地层进行压力预测的限制，可以对砂泥岩地层开展地层压力预测；同时克服了原模型卸载方程中当有效应力无限趋于0，声波波速趋于海水波速的理论缺陷。现场应用表明，改进的Bowers地层压力预测模型在乐东区域的预测结果平均误差仅为2.6%，该模型在乐东区域具有良好的适用性。

A method for predicting the pressure of unloading-type abnormal pressure formation

Ledong area in the South China Sea is a typical unloading-type high pressure area, where the conventional formation pressure prediction method is poorly applicable and on-site drilling operation faces the risks of well kick and well blowout. In order to establish a formation pressure prediction model applicable to Ledong area, this paper carried out laboratory acoustic testing experiments on artificial cores and established the functional relations between acoustic velocity and effective stress, lithology and shale content. It is experimentally indicated that acoustic velocity is in an exponential relation with effective stress and in a linear relation with the square root of porosity and shale content. Then, the modified Bowers formation pressure prediction model was established according to the linear superposition principle. The original model can be only used to predict the pressure of mud shale formations, but the modified Bowers model breaks this restriction and can be also used to predict the pressure of sandy mudstone formations. In addition, the modified Bowers model overcomes the theoretical defect of the unloading equation in the original model that the acoustic velocity approaches to the wave velocity of sea water when the effective stress approaches to zero infinitely. The field application shows that the average error of the prediction result in Ledong area by modified Bowers formation pressure prediction model is only 2.6%, indicating its good applicability in Ledong area.